His Popular Party is expected to take the most votes, but he is likely to have trouble forming a government

MADRID—As Spain girds for its second national election in just over six months, the campaign logo for the front-running Popular Party has attracted attention for what it doesn’t feature—any reference to its standard-bearer, Prime Minister Mariano Rajoy.

Popular Party officials say the simple logo bearing the party initials and the slogan “A Favor,” (In Favor), reflects their choice of an austere campaign, not an effort to deflect attention from Mr. Rajoy, whose image has been hammered by high unemployment and corruption scandals.